1. Field of the Invention
The invention relates to a method for halftone reproduction of continuous tone images, in black and white or color, without the use of halftone screens, whereby improved tonal density characteristics are achieved.
2. Description of the Prior Art
As is well known, conventional halftone black and white reproductions of continuous tone images, such as photographs, drawings, and other graphic works, are formed by reproducing the image with dots of various sizes. The usual halftone reproduction process utilizes a haltone screen placed in front of a photographic film in a copying camera. In such an arrangement, those areas of high light intensity are passed through the screen essentially unimpaired causing small dots to be formed on a photographic film. Lesser light intensities are impeded proportionately, causing larger dots to be formed. In this manner, black, white, and the various tones of gray in between may be reproduced upon the photographic film, which is then processed in a conventional manner to produce halftone printing plates.
Halftone printing plates have a surface consisting of thousands of uniformly placed dot protrusions of various sizes. After printing these dots on paper, the eye, not being sufficiently microscopic to see them as dots, blends the dots into the various tones of the continuous tone image being reproduced.
For forming halftone color reproductions of continuous tone color images, the four-color process is generally used. In this process, by means of photographic filters, color separation negatives are made for each of the primary colors -- yellow, red, and blue -- and for black. From these in turn, halftone printing plates are made for each color in a conventional manner. By inking the plate with the appropriate color and applying each of these plates upon the same sheet of paper, the desired halftone image is printed.
In making color halftone printing plates, a halftone screen is used by means of which the screen angle -- the angle made by the lines of the screen with the horizontal -- is changed for each color. This is done so that the dots for the four colors, when printed along the lines of their respective screens, will tend to lie alongside one another, not on top of one another, thereby avoiding moire, or patterning interference, and enabling the eye to blend them into secondary or tertiary colors. Where blue and yellow dots, for example, fall side by side, the eyes see them as a green, and this may be a blue-green or a yellow-green according to predominance of blue or yellow dots.
Halftone color reproduction may also be accomplished by a three-color process, in which the colors used are yellow, red, and a blue, darker than that used in the four-color process.
With the advent of electrical reproducing apparatus, various approaches have been suggested for achieving halftone reproduction of continuous tone images in order to provide a more efficient system to minimize the time and expense associated with the previously known methods of halftone reproduction. Such approaches have sought to acquire better rendition of tonal levels, or tonal density characteristics, and the elimination of the previously required screening step.
U.S. Pat. No. 3,742,129, issued to Roberts et al, is one example of a method and apparatus for half-tone reproduction of continuous tone images, particularly of color images, which electrically generates halftone dot characters. The Roberts et al patent employs a cathode ray tube and accompanying circuitry for deflecting the cathode ray beam along the CRT face for purposes of generating alphanumeric characters or graphical images in halftone dot format. These characters or images appear as illuminated portions on the face of the cathode ray tube and are recorded on a photosensitive film which is processed for purposes of making halftone printing plates. The Roberts et al device utilizes character fields which are divided into forty-five squares, wherein from zero to forty-five squares may have a dot formed therein. The size of the characters, which is the number of dots formed within the character field, determines the gray tone level being reproduced. Therefore, from zero to forty-five dots may be formed within a character field, thus making possible the rendition of forty-six gray tone levels. For reproduction of continuous tone color images, the Roberts et al device generates halftone screen patterns for each of the primary colors in accordance with the various equations set forth in the Roberts et al patent.
U.S. Pat. No. 3,604,846, issued to Behane et al is another example of a method and apparatus for halftone reproduction. Instead of using a CRT to form dot characters, this patent discloses apparatus for placing ink dots upon a receiving member such as a sheet of paper. As in Roberts et al, the number of gray-tone levels or tonal densities is determined by the number of dots placed within a grid or character field. Within each grid, Behane et al discloses the use of a 3 .times. 3 up to a 5 .times. 5 matrix, thus making possible from ten to twenty-six different tonal densities. The Behane et al patent further discloses the use of its apparatus for reproducing color separations normally used in the production of color printing plates. Although the disclosure to this feature is not very detailed, it appears that the color separations being reproduced have previously been subjected to the conventional color screening step before being reproduced by the Behane et al method and apparatus.
Many other patents are to be found in the prior art for electronic halftone reproduction of continuous tone images; however, the available number of tonal densities which may be achieved is dependent upon the size or number of the dots being formed and, if color reproduction is disclosed, either a color screening step or its equivalent must be performed; e.g., U.S. Pat. No. 3,671,666, issued to Hennig, U.S. Pat. No. 3,688,036, issued to Odnolko et al, and U.S. Pat. No. 3,725,574, issued to Gast.
A major problem encountered in using these prior art methods and apparatus for halftone reproduction of continous tone images has been the limited number of tonal densities which can be reproduced. Although the Roberts et al patent discloses the reproduction of up to forty-six tonal densities, it is desirable to increase the number of tonal densities which may be obtained. This is particularly true in view of the fact that many available scanning apparatus are able to determine as many as 250 or more tonal densities from a continuous tone image. Additionally, it would be desirable to be able to use a dot or character field which is not as complex as that of the Roberts et al patent.
Another shortcoming of the prior art methods and apparatus has been the necessity and expense of the required color screening step performed prior to producing suitable printing plates.
Accordingly, prior to the development of the present invention, there has been no method available for halftone reproduction of continuous tone images which can produce an increased number of tonal densities and, for color reproduction, does not require a color screening step. Therefore, the art has sought an efficient method for halftone reproduction of continuous tone images in color, and black and white, absent the problems of previously proposed methods and appartus.